1. Field of the Invention
This invention relates to processes for producing an electrophotgraphic toner, and more specifically, to processes for producing a toner containing a coloring agent and having a suitable particle diameter and a uniform particle size distribution in one step by polymerizing monomers capable of forming a fixing resin in suspension.
2. Description of the Prior Art
In electrophotography, a toner is used to develop a latent electrostatic image to a visible toner image. The toner particles are composed of a composition comprising a resin medium and a coloring agent and as required, a charge controlling agent and other additives and having a certain particle diameter range of, for example, 1 to 30 .mu.m. Resins having the desired chargeability and bindability, such as styrene-type resins, are used as the resin medium, and carbon blacks and other organic or inorganic colored pigments are used as the coloring agent.
Most typically, electrophotographic toners are produced by a process which comprises melt-kneading the resin medium and the coloring agent, cooling and pulverizing the resulting melt-kneaded composition, and classifying the pulverized particles to adjust their size to a fixed range. The yield of the toners obtained by pulverization and classification, however, is low, and much equipment is required for these operations. Hence, the cost of producing toners is very high. Furthermore, since the resulting toner particles have irregular shapes, their flowability is generally low and blocking tends to occur.
Many proposals have previously been made to produce toners directly in a step of forming a toner resin by polymerization. A typical example is a process whch comprises dissolving a soluble polymerization initiator in a water-insoluble monomer, adding additives such as a coloring agent, suspending the resulting composition in an aqueous solution containing a suitable dispersing agent, such as a water-soluble polymer, an inorganic powder or a surface-active agent, with stirring under high shear, and polymerizing the suspended monomer particles to produce colored polymer particles.
In this suspension polymerization process, the size of the final toner particles is determined by the state of suspension of the monomeric composition in water. The distribution of the sizes of the particles is very broad because of the state of putting the monomeric composition in water and agitating it, and it is difficult to obtain toner particles having a single particle diameter. Furthermore, this suspension polymerization process only gives coarse particles having a particle diameter of several tens of .mu.m to several mm, and it is difficult to obtain particles having a size of 1 to 30 .mu.m which are useful as a developing toner. Of course, the particle size of the resulting toner can be rendered fine by increasing the amount of the dispersing agent incorporated. In this case, the dispersing agent is included in the toner, and the toner is liable to be sensitive to humidity and have degraded electrophotographic properties. To prevent this, a special after-treatment operation is necessary and the number of the required steps is too many to be practical.
The problem of the toners obtained by the suspension polymerization process is that the inclusion of a certain proportion of fine particles is difficult to avoid, and their particle size is non-uniform. Such fine particles are particles of the polymer itself not containing toner additives such as the coloring agent. Since such fine particles have different characteristics from the toner particles, they become a cause of toner scattering or fogging during development even when their amount is small. Such fine particles also tend to reduce the flowability of the toner particles. Even if such fine particles can be removed, the resulting toner particles have a greatly different composition form that initially formulated. The desired electrophotographic properties cannot be obtained, and the reduced yield of the toner particles leads to the increased cost of production.